Fix segfault sorting runnable steps

Same problem as d744362e4a249a92ecfc3e48eae135e3a46db49e.

at /nix/store/ksvsbr7pg4z69bv6fbbc8h7x7rm2104m-gcc-4.9.3/include/c++/4.9.3/bits/predefined_ops.h:166
__last@entry=..., __comp=...) at /nix/store/ksvsbr7pg4z69bv6fbbc8h7x7rm2104m-gcc-4.9.3/include/c++/4.9.3/bits/stl_algo.h:1827
__comp=...) at /nix/store/ksvsbr7pg4z69bv6fbbc8h7x7rm2104m-gcc-4.9.3/include/c++/4.9.3/bits/stl_algo.h:4717

Created by  Eelco Dolstra  on March 2, 2016
PDJCVDVY6R4ZCBELQR3KXPJ3LE7TVXG6N74USR7HWBOIE2P6HDAAC
Change contents